Method for Detecting Anomalies on a Submarine Object

ABSTRACT

A method for detecting anomalies on a submarine object, in particular in the submarine region on a hull of a moored warcraft, which method carries out very reliable sensing of the submarine object by way of an unmanned small submarine vehicle that is equipped with simple sensor equipment, such as an acoustic sensor for measuring distances and a barometric cell for determining depth, and which method obtains a profile of the submarine object by navigating the small submarine vehicle with a constant transversal distance to the submarine object, in which profile an anomaly present on the submarine object becomes is apparent from the profile line.

The invention relates to a method for detecting anomalies on anunderwater object, in particular on the underwater part of a hull of asubmerged watercraft, according to the precharacterizing clause of claim1.

During times of increasing terrorist threat to civilian and militaryfacilities, their permanent protection is becoming increasinglyimportant. Underwater objects in particular, such as foundations of feedinstallations and wind farms, docks, marine-vessel hulls ofmarine-vessels and submarines in harbor, are subject to underwatermanipulation by divers or remotely controlled underwater vehicles,without protection. For example, limpet mines can be fitted withoutbeing noticed, and are remotely fired.

DE 10 2005 014 555 A1 discloses a mine hunting system and a method formine hunting using a plurality of autonomously acting underwatervehicles, with a first group of these underwater vehicles, which havesensors, being used for mine location, and a second group of theseunderwater vehicles being used to attack mines that have been located.

US 2009/0090286 A1 discloses an armed, remotely controlled craft withvideo and sonar sensors.

Furthermore, DE 10 2005 062 109 A1 discloses a method and an apparatusfor defense against personnel ingressing underwater with a person firstof all being detected and tracked, and with an unmanned underwatervehicle then being deployed for defense against the person detected.

Furthermore, DE 43 02 455 A1 discloses an underwater drone for attackingmines, with this underwater drone having an antenna device which issuitable for metal detection.

Finally, DD 300 802 A7 discloses an underwater body with a fixedarrangement of hydroacoustic transducers for basic distance measurementwhich can be used universally in three operating modes.

The invention is based on the object of specifying a cost-effectivemethod for detecting or identifying anomalies on underwater objects, forexample of foreign bodies illegally fitted there, such as limpet mines,smuggled goods and the like, which is efficient and can be carried outin a largely automated form without having to use personnel underwater.

According to the invention, the object is achieved by the features inclaim 1.

The method according to the invention has the advantage that theunderwater object can be scanned very reliably by a single sensorfitting, such as an acoustic sensor for lateral distance measurement anda pressure capsule for depth determination, and a profile of theunderwater object is attained by navigation of the small underwatervehicle at a constant lateral distance from the underwater object, inthe profile line of which profile an anomaly which exists on theunderwater object, for example a foreign body which has been stuck on,is clearly evident. This is a result of the fact that, when the anomalyis detected by the acoustic sensor, the small underwater vehiclecontinues its movement, which is characterized by a constant lateraldistance from the underwater object, because of its inertia, while incontrast, however, a profile change appears in the measurement profileof the acoustic sensor which measures the lateral distance, for examplea notch or dip in the profile line, which has already decayed again whenthe movement control system for the small underwater vehicle is causedto react to the change in the lateral distance by the navigationapparatus, as a result of which the small underwater vehicle continueson its course at a predetermined, constant lateral distance from theunderwater object, without any change, independently of the brief changein the lateral distance from the underwater object. The identificationof the anomaly in the measurement profile line of the acoustic distancesensor can be moved directly, for example via an optical waveguide towedbehind the small underwater vehicle, for warning indication in amonitoring center and can be used to initiate a diver operation forinspection and/or removal of the anomaly, without the small underwatervehicle having to interrupt or terminate its inspection movement. Thisresults in a considerable time saving between identification of theanomaly, and its removal.

Expedient embodiments of the method according to the invention, togetherwith advantageous developments and refinements of the invention, arespecified in the further claims.

According to one advantageous embodiment of the method, the position ofthe small underwater vehicle with respect to the underwater object isfound at least on identification of an anomaly. This determination ofthe position of the small underwater vehicle allows the anomaly to belocated on an object-related basis easily, and the inspection and/orremoval operation to be carried out by divers in a shorter timescale,and efficiently.

According to one advantageous embodiment of the invention, the smallunderwater vehicle moves at a constant velocity, and the time of travelis measured continuously during movement. When an anomaly is identified,the position of the anomaly is determined from the time of travelmeasured until then and the velocity of travel of the small underwatervehicle and the submersion depth of the small underwater vehicle. Thetime measurement is started when the predetermined lateral distancebetween the small underwater vehicle and the underwater object ismeasured for the first time. This procedure allows the position of thesmall underwater vehicle, and therefore the object-related position ofthe anomaly, to be detected by a simple time measurement.

According to one advantageous embodiment of the invention, a repeatedmovement away from the underwater object is carried out and, on eachmovement away, the constant movement depth is changed. This movementaway from the underwater object at different submersion depths alsoallows the depth component of the anomaly to be determined sufficientlyaccurately for diver operation using a single acoustic sensor withlittle vertical shaping of the acoustic scanning beam. In this case, themovement depth change of the small underwater vehicle can be carried outimmediately at the end of the underwater object by a 180° reversing turnof the small vehicle or after moving completely around the underwaterobject.

The method according to the invention will be described in more detailin the following text with reference to one exemplary embodiment whichis illustrated in the drawing, in which, illustrated schematically:

FIG. 1 shows a side view of a surface vessel moored in a dock,

FIG. 2 shows a plan view of the surface vessel in FIG. 1,

FIG. 3 shows a diagram of the lateral distance between the smallunderwater vehicle and the hull of the surface vessel as a function ofthe time of travel when the hull of the surface vessel in FIG. 1 andFIG. 2 is scanned by an acoustic distance sensor during movement of thesmall underwater vehicle,

FIG. 4 shows a block diagram of an apparatus for carrying out the methodfor detecting or identifying an anomaly on the hull of the surfacevessel in FIGS. 1 and 2.

In order to explain the method proposed here for detecting, identifyingor discovering anomalies on a stationary underwater object, FIGS. 1 and2 schematically illustrate a surface vessel 11 which is located in adock 12 and is moored to a pier 13, that is to say it is held firmly bylines 14.

An anomaly 16 which, for example, may be a limpet mine or a containerfilled with smuggled goods is illustrated in the underwater area on thehull 15 of the surface vessel 11.

An unmanned small underwater vehicle 17 is used in order to detect ananomaly 16 such as this on the otherwise smooth hull 15 of the surfacevessel 11. Such unmanned, self-powered small underwater vehicles areknown in many forms with different sensor and measurement apparatusfits. The small underwater vehicle 17 used here has, for example, fourpropeller drives 18, which are operated separately by a navigationapparatus 19 (FIG. 4) in order to control or navigate the smallunderwater vehicle 17. Depending on the rotation speed of the individualpropeller drives 18, of which in each case two are arranged verticallyone above the other and two horizontally alongside one another, thesmall underwater vehicle 17 can move straight ahead or can be steered tothe right or left, and upward or downward. At least one acousticdistance sensor 20 for measurement of a lateral distance which extendshorizontally with respect to the craft axis and a depth sensor 21 fordetermining the submersion depth of the small underwater vehicle 17 areprovided as sensors in the small underwater vehicle 17 used here. By wayof example, an acoustic distance sensor 20 such as this may be a simpleecho sounder which emits sound pulses and receives the echoes resultingfrom reflection of the sound pulses, measuring the time betweentransmission and echo reception. The distance to the object causing thereflection of the sound pulses is calculated from the measured time,taking account of the speed of sound. By way of example, the depthsensor 21 is a simple pressure capsule. The output signals from thesensors 20, 21 are supplied to the navigation apparatus 19.

The small underwater vehicle vehicle 17 is deployed into the water fromthe surface vessel 11 or from the pier 13, for example—as is illustratedin FIGS. 1 and 2—behind the stern of the surface vessel 11, and movesalong the hull 15 of the surface vessel 11. During the movement of thesmall underwater vehicle 17, the acoustic distance sensor 20continuously measures the horizontal lateral distance between the smallunderwater vehicle 17 and the hull 15. In this case, the smallunderwater vehicle 17 is controlled by the navigation apparatus 19 suchthat it maintains a predetermined lateral distance from the hull 15, ata constant depth. For this purpose, on the one hand, the movement depthand the lateral distance are predetermined as nominal values T_(nom) andd_(nom) of the navigation apparatus 19 (FIG. 4), and on the other handthe measured values are supplied from the distance sensor 20 and thedepth sensor 21 as actual values d and T. An appropriate control loop inthe navigation apparatus 19 generates control commands as a functionthereof, for the four propeller drives 18, which keep the smallunderwater vehicle 17 on the course that has been mentioned.

The actual lateral distance d which is measured continuously by thedistance sensor 20 during the movement of the small underwater vehicle17 is furthermore compared continuously with the predetermined nominallateral distance d_(nom) and, if the actual lateral distance d fallssignificantly below the nominal lateral distance d_(nom), the presenceof an anomaly is identified. The small underwater vehicle 17 ispreferably connected via a connecting line 22 (FIG. 1) to a missionmonitoring center on board the surface vessel 11 and, when an anomaly isdetected, an alarm can be triggered via the connecting line 22. In thiscase, at the moment when the anomaly is detected, the instantaneousposition of the small underwater vehicle 17 relative to the hull 15 ofthe surface vessel 11 is also determined, and is signaled via theconnecting line 22 to the mission monitoring center. The position isdetermined in a simple manner by the small underwater vehicle 17 movingat a constant velocity, which is predetermined as the nominal velocityvalue v_(nom), for the navigation apparatus 19, and in that the time oftravel t is measured from a starting point. The time of travel t_(A)measured at the time when the anomaly 16 is identified results in thedistance s_(A) traveled at the predetermined submersion depth T_(nom),and the position P_(A) (S_(A); T_(A)) of the small underwater vehicle 17is thus found. If the time t_(o) at which the distance sensor 20measures a lateral distance d which corresponds to the predeterminednominal lateral distance d_(nom) for the first time after the start ofoperation of the small underwater vehicle 17 is chosen as the startingtime for the time measurement, then the output position P_(A) (S_(A);T_(A)) of the small underwater vehicle is at the same time the positionof the anomaly 16 on the hull 15 of the surface vessel 11.

By way of example, FIG. 4 shows a block diagram of an apparatus which isinstalled in the small underwater vehicle 17, and by means of which theproposed method for detecting or discovering the anomaly 16 is carriedout. In addition to the navigation apparatus 19 which has already beenmentioned and the sensors 20, 21 which have already been mentioned, theapparatus also has a first flank detector 23, a timer 24, a comparator25, a second flank detector 26, a gate circuit 27 and a multiplier 28.The output of the acoustic distance sensor 20 is connected both to thenavigation apparatus 19 and to the inputs of the flank detectors 23, 26and of the comparator 25. Via a second input, the comparator 25 issupplied with the predetermined lateral distance d_(nom) between thesmall underwater vehicle 17 and the hull 15 of the surface vessel 11.The gate circuit 27 can be operated via the output of the comparator 25,and connects the output of the timer 24 and the input of the multiplier28, to which the nominal velocity v_(nom) of the small underwatervehicle 17 is supplied as a multiplier. The outputs of the two flankdetectors 23, 26 are connected to the navigation apparatus 19.

In order to illustrate the proposed method, FIG. 3 shows a diagram inwhich the lateral distance d measured by the acoustic distance sensor 20during the movement of the small underwater vehicle 17 is illustrated asa function of the time of travel t. The small underwater vehicle 17,which is deployed behind the stern of the surface vessel 11, starts tomove, and, at the time t₀, arrives at a constant velocity and at thesubmersion depth T_(nom) at the stern end of the hull 15. During thismovement, the acoustic distance sensor 20 carries out measurementsagainst the pier wall, and therefore measures the lateral distance dfrom the pier wall, which is considerably greater than the nominal valued_(nom) predetermined for the navigation apparatus 19. When the smallunderwater vehicle 17 reaches the hull 15 of the submarine 11, then asignificant certain measured-value change occurs at the output of theacoustic distance sensor 20, since the lateral distance d which is nowmeasured by the distance sensor 20 against the hull 15 is very much lessthan the lateral distance previously measured against the pier wall.This negative certain measured-value change leads to a control pulse atthe output of the first flank detector 23, which control pulse on theone hand switches on the distance control loop of the navigationapparatus 19, and on the other hand starts the timer 24. The smallunderwater vehicle 17 is now controlled on a course on which the smallunderwater vehicle 17 maintains a constant predetermined lateraldistance d_(nom) from the hull 15.

At the time t_(A), the small underwater vehicle 17 arrives at theanomaly 16 on the hull 15, and the output signal from the acousticdistance sensor 20 falls briefly below the nominal value d_(nom). Apulse occurs at the output of the comparator 25, which continuouslycompares the actual value, which is output from the distance sensor 20,of the lateral distance d from the hull 15 with the predeterminednominal value of the lateral distance d_(nom), which pulse causes thegate circuit 27 to briefly close. In consequence, the time of travelt_(A) measured at that time by the timer 24 is passed to the multiplier28. In the multiplier 28, the time of travel t_(A) detected at that timeis multiplied by the predetermined nominal velocity v_(nom) of the smallunderwater vehicle 17. The distance s_(A) of movement obtained from thiswhich, together with the predetermined submersion depth T_(nom) of thesmall underwater vehicle 17, defines the position of the smallunderwater vehicle 17 at the moment when the anomaly 16 is detected canbe transmitted via the connecting line 22 to the mission monitoringcenter on board the surface vessel 11, where it is integrated in analarm indication. On the basis of the alarm indication, the monitoringcenter can start a guide operation for inspection and removal of theanomaly 16, with the position of the small underwater vehicle 17determined from the signaled distance s_(A) of movement and the signaledsubmersion depth T_(nom) indicating the position P_(A) of the anomaly16, which forms the preset target for the diver operation.

Independently of this, the small underwater vehicle 17 continues itsmovement at a constant lateral distance d_(nom) from the hull 15 of thesurface vessel 11. When the small underwater vehicle 17 has reached theend of the hull 15 and moves beyond this, then the acoustic distancesensor 20 once again measures the lateral distance from the pier wall,which is considerably greater than the lateral distance from the hull15. A significant certain measured-value change to higher measuredvalues occurs at the output of the distance sensor 20. The positiveflank of the certain measured-value change is detected in the secondflank detector 26. The latter produces a control pulse, which is passedto the navigation apparatus 19, where it initiates a movement of thesmall underwater vehicle 17, for example a turning maneuver to adifferent submersion depth.

The described process of the small underwater vehicle 17 moving awayfrom the hull 15 is carried out repeatedly at a different submersiondepth of the small underwater vehicle 17, in such a way that the entirehull 15 is also scanned completely by the acoustic distance sensor 20 inthe vertical dimension. After leaving the hull area, it is sensible forthe small underwater vehicle to carry out a 180° turn, and to move awayfrom the hull 15 at the next submersion depth, in the opposite directionto its previous movement path. In this case, the small underwatervehicle 17 must be equipped with a second acoustic distance sensor,whose measurement direction is rotated through 180° with respect to thatof the first acoustic distance sensor 20.

All of the features mentioned in the above description of the figures,in the claims and in the introductory part of the description can beused both individually and in any desired combination with one another.The invention is therefore not restricted to the combinations offeatures described and claimed. In fact, all combinations of featuresshould be considered as having been disclosed.

1. A method for detecting anomalies (16) on an underwater object, inparticular in the underwater area of a hull (15) of a submergedwatercraft, wherein: an unmanned small underwater vehicle (17), which isequipped with a navigation apparatus (19) and an acoustic sensor whichmeasures transversely with respect to the direction of travel, moves ata constant depth (T_(nom)) along the underwater object and in theprocess the acoustic sensor continuously measures the lateral distancebetween the small underwater vehicle (17) and the underwater object; thenavigation apparatus (19) controls the small underwater vehicle (17)such that the small underwater vehicle (17) maintains a constantpredetermined lateral distance (d_(nom)) from the hull (15); and themeasured lateral distance (d) is continuously compared with thepredetermined lateral distance (d_(nom)), and an anomaly (16) on theunderwater object is identified if there is a significant discrepancy.2. The method as claimed in claim 1, wherein a repeated movement awayfrom the underwater object is carried out and, on each movement away,the constant movement depth (T_(nom)) of the small underwater vehicle(17) is changed.
 3. The method as claimed in claim 2, wherein theposition of the small underwater vehicle (17) with respect to theunderwater object is found at least on identification of an anomaly. 4.The method as claimed in claim 3, wherein the small underwater vehicle(17) moves at a constant velocity (v_(nom)) and the time of travel (t)is measured continuously, and in that when an anomaly is identified, thehorizontal component of the position of the anomaly (16) is determinedfrom the time of travel (t_(A)) measured until then and the velocity oftravel (v_(nom)) of the small underwater vehicle (17).
 5. The method asclaimed in claim 1, wherein the position of the small underwater vehicle(17) with respect to the underwater object is found at least onidentification of an anomaly.
 6. The method as claimed in claim 5,wherein the small underwater vehicle (17) moves at a constant velocity(v_(nom)) and the time of travel (t) is measured continuously, and inthat when an anomaly is identified, the horizontal component of theposition of the anomaly (16) is determined from the time of travel(t_(A)) measured until then and the velocity of travel (v_(nom)) of thesmall underwater vehicle (17).